As a waste water treatment method, there is a membrane separation activated sludge method comprising immersing a membrane cartridge in an activated sludge tank and carrying out solid-liquid separation of the activated sludge and the treated water after the treatment. This method allows the concentration of the activated sludge (MLSS: Mixed Liquor Suspended Solid) to be set at a very large value from 5,000 to 20,000 mg/l for a filtering process. This advantageously allows the capacity of the activated sludge vessel to be reduced or enables a reaction time in the activated sludge vessel to be shortened. Further, the filtration with the membrane prevents suspended solids (SS) from being mixed into treated water, thus eliminating the need for a final sedimentation tank. This makes it possible to reduce the construction area of the treatment facility and to achieve filtration regardless of whether or not activated sludge is appropriately sedimented. This method thus has advantages such as a reduction in the load of activated sludge managing operation. Therefore, in recent years, the membrane separation activated sludge method has prevailed rapidly.
If hollow fiber membranes are used for the membrane cartridge, the high strength of the membrane itself hinders the surface of the membrane from being damaged as a result of contact with contaminants contained in the raw water. The membrane cartridge can thus be used for a long period. Moreover, this structure has the advantage of being capable of back wash reverse filtration, that is, injecting a medium such as treated water in a direction opposite to that of filtration to remove fouling to the membrane surface. In this case, however, effective membrane area may decrease unless filtration is carried out while excluding aggregates of activated sludge as well as contaminants from the raw water accumulating in the gap between the hollow fiber membranes. As a result, filtration efficiency lowers, thus stable filtration is prevented from being maintained over a long period.
Conventionally, the following method is used to avoid accumulating sludge or the like on the surfaces of hollow fiber membranes or between the hollow fiber membranes. That is, aeration by air or the like is performed from the lower portion of a membrane cartridge, and activated sludge aggregate and contaminants brought from raw water on the surface of a hollow fiber membrane or between hollow fiber membranes are removed in accordance with the oscillation effect of a membrane and agitation effect by movement of bubblers upward and thereby, accumulation of them is prevented. For example, a lower ring (or also referred to as skirt) is set to the lower portion of a hollow fiber membrane cartridge and a plurality of through-holes are formed on a lower-ring-side adhesion fixation layer to form an air pool in the end of the lower ring protruded from the lower ring by aeration from the lower portion of the cartridge. Thus, bubbles are uniformly generated from the plurality of through-holes and hollow fiber membranes are oscillated so that a suspended material deposited on the outer surfaces of the membranes are easily removed. (For example, refer to Patent Document 1).
According to this method, when filtering high-concentration MLSS such as the case of the membrane separation activated sludge method, there is an effect for removing the sludge between hollow fiber membrane bundles in accordance with the agitation effect by aeration and the oscillation effect of a membrane. However, a force for raising activated sludge aggregate or contaminants acts due to ascent of bubbles, and removed sludge moves to a position nearby the adhesion fixation layer, and they are not easily removed to the outside of the membrane bundle. Moreover, the hollow fiber membrane at the portion nearby the adhesion fixation layer has a small oscillation amplitude and therefore, it is impossible to sufficiently remove fouling from the surface of the membrane, and there is a problem that the surface of a hollow fiber is clogged because sludge is accumulated between membranes.
[Patent Document 1] JP-A-2000-157846